Device for producing SiC single crystals

ABSTRACT

SiC single crystals are produced in a reaction chamber, in which there is a seed crystal for the separation of a SiC single crystal from the gas phase. The reaction chamber is connected to a storage chamber, which is at least partly filled with a supply of SiC, by a gas channel with a predetermined cross-section for conveying the SiC in the gas phase. The supply of SiC is sublimated in a heating device and a temperature gradient is adjusted in the reaction chamber. It is, thus, possible to produce SiC single crystals with any desired cross-sectional area and of high crystalline quality and single-crystal yield, because the conveyance rate of the gas molecules can be precisely adjusted.

BACKGROUND OF THE INVENTION

The invention relates to a device and a process for producing singlecrystals from silicon carbide (SiC).

A known process for producing SiC single crystals is the sublimation oftechnical grade SiC in powder form and growing of this SiC out of thegas phase on a monocrystalline SiC seed crystal. In the case of a firstknown device for implementing such a process, a cylindrical reactionvessel is provided in a vacuum installation, and the outer wall of thisvessel surrounds a hollow cylindrical heating wall and an upper andlower heating plate. The heating wall and the heating plates consist ofelectrographite and are inductively coupled to a high-frequency (HF)heating coil arranged outside of the vacuum installation. A hollowcylindrical cylinder wall made of porous graphite is arranged inside theheating wall, concentrically to the heating wall. This intermediate wallseparates a likewise hollow cylindrical supply chamber between theintermediate wall and the heating wall from a cylindrical reactionchamber inside the intermediate wall. A flat SiC seed crystal isarranged in the lower part of the reaction chamber symmetrically to thecylinder axis. A SiC supply that fills the supply chamber is heated bymeans of the heating wall and the heating plates to a temperature ofabout 2000° C. to about 2500° C., and the solid SiC is sublimated. Thegas mixture that is formed thereby from the main components Si, Si₂ Cand SiC₂, also referred to in the following as "SiC in the gas phase"diffuses through the pores of the graphite into the upper part of thereaction chamber and, from there, to the seed crystal that is retainedat a crystallization temperature of about 1900° C. to 2200° C. The SiCcrystallizes out on the seed crystal. The temperature gradient betweenthe upper part and the lower part of the reaction chamber is adjusted,at the most, to 20° C. /cm, in that an additional thermal insulationand/or an additional heating is provided for the upper heating plate andan additional cooling is provided for the seed crystal. Moreover, aprotective gas, preferably argon (Ar) is fed into the reaction chamberto adjust a pressure of about 1 to 5 mbar, which counteracts the vaporpressure of the SiC in the gas phase. With such a device, it is possibleto produce SiC single crystals having a length of at least 30 mm and adiameter of up to 40 mm (German C-32 30 727).

In the case of another known device, in place of a common HF-coiloutside of the vacuum vessel, two resistance heaters are disposed insideof the vacuum vessel. One of these two resistance heaters is providedfor heating a powdery SiC supply in a supply chamber to a sublimationtemperature of typically about 2300° C., and the other resistance heateris provided for heating the crystallization surface on a seed crystaldisposed in a reaction chamber to a crystallization temperature oftypically 2200° C. The reaction chamber is disposed in this case abovethe supply chamber and separated from it by a separating wall of porousgraphite. The temperature of the SiC powder and the temperature at thecrystallization surface can be controlled independently of one anotherduring the manufacturing process by means of the two resistance heaters,which are independent of one another. Therefore, the temperaturegradient between the SiC powder in the supply chamber and thecrystallization surface in the reaction chamber evolves by itself whenthese two temperatures are specified in dependence upon the thermalproperties of the system, in particular upon the thermal transitioncoefficients of the materials and its geometry. The growth process ofthe single crystal growing on the seed crystal can be positivelyinfluenced by this independent adjustment of the sublimation temperatureand of the crystallization temperature. To adjust a nearly constanttemperature gradient during the crystal growth between thecrystallization surface on the growing single crystal and the SiCpowder, whose volume is decreasing during the process, the seed crystaland the growing single crystal are disposed on a shaft so as to beaxially movable toward or away from the SiC powder surface. Moreover,this shaft is rotatable, so that a dynamically balanced growth isachieved and spatial fluctuations in the gas flow are averaged out. SiCcrystals of the 6H modification having a 12 mm diameter and 6 mm heightwere produced using such a device (U.S. Pat. No. 4,866,005).

Due to surface reactions at the surface of the pores in the graphite ofthe intermediate wall, one cannot avoid impurities in the grown SiCsingle crystal when known methods are used. Moreover, the pore size ofthe graphite of the intermediate wall is subject to relatively largefluctuations, depending on the manufacturing process. Consequently, theconveyance of gas through the pores and, thus, also the growth rate ofthe monocrystalline SiC are not able to be exactly adjusted. Finally,only one single monocrystal can be produced per deposition process,since it is necessary for the seed crystal and, thus, the grown singlecrystal to be arranged in the area of the axis of symmetry due to theradial input of heat from the outside and the axial dissipation of heatto the outside.

This invention is directed to a device to enable the three essentialpartial processes, sublimation, gas conveyance and crystallization, tobe better controlled and to a process for producing SiC single crystals,so as to allow the crystal quality, the single crystal yield and thesize of the single crystals to be increased. Moreover, an object of thepresent invention is to enable a plurality of single crystals to begrown at the same time.

SUMMARY OF THE INVENTION

The present invention solves this problem by providing a supply chambercontaining the SiC supply and a reaction chamber containing the seedcrystal. These chambers are disposed so as to be spatially separatedfrom one another and are connected to one another by a gas channelhaving a well defined cross-section. By this means, the geometry and, inparticular, the volumes of the supply chamber and of the reactionchamber can be selected independently of one another and any desiredcrystal diameters can be achieved in contrast to thecylindrical-concentric arrangement in the case of the previouslydescribed Prior Art. Furthermore, given a preselected pressure and apreselected temperature distribution within the system, the particlestream of the gas molecules of the SiC in the gas phase can be adjustedboth with respect to its size as well as its direction by the dimensionsand arrangement of the gas channel.

Advantageous developments in accordance with the invention are revealedby each of the dependent subordinate claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of an example of the device forproducing SiC single crystals according to an embodiment of the presentinvention, including a supply chamber arranged above the reactionchamber;

FIG. 2 is a cross sectional view of another embodiment of the presentinvention, including a supply chamber arranged below the reactionchamber;

FIG. 3 is a cross sectional view of a third embodiment of the presentinvention including a homogenization chamber arranged between the supplychamber and the reaction chamber and connected with these chambers by agas channel;

FIG. 4 is a cross sectional view of a fourth embodiment of the presentinvention including a plurality of systems, which are composed in eachcase of a supply chamber, a reaction chamber, and a gas channel andwhich have a common heating system; and

FIG. 5 is a cross sectional view of a fifth embodiment of the presentinvention including a plurality of reaction chambers, which are eachconnected via a gas channel to a common supply chamber, and comprising acommon heating system.

DETAILED DESCRIPTION

The same parts are given the same reference numerals in each case. InFIG. 1, a reaction chamber is designated by 2, a seed crystal by 21, itsseed chuck by 22, a supply chamber by 4, a supply of solid SiC by 40, aretaining device for the supply 40 by 42, a separating wall by 8, a gaschannel by 3, whose intake port is denoted by 3B and exhaust port by 3A,as well as whose center axis by 3C, and a heating device by 6. In itslower part, the supply chamber 4 is filled with the supply 40. Thesupply 40 is preferably comprised of technical grade SiC in a powderform or of solid, generally polycrystalline SiC, and can also containdopants for doping the SiC single crystal. The reaction chamber 2 isarranged below the supply chamber 4. The supply chamber 4 and thereaction chamber 2 are separated from one another by the impermeableseparating wall 8 and interconnected by the gas channel 3, which has adefinitively preselected cross-section and runs through the separatingwall 8. Thus, it is possible to adjust the gas conveyance rate and alsothe direction of the gas particle stream. The cross-section of the gaschannel 3 is generally selected to be between 0.05 mm² and 200 mm² and,preferably, between 0.1 mm² and 100 mm² and can vary along the length ofthe gas channel 3 in a specified manner.

To ensure that solid SiC cannot reach the reaction chamber 2 from thesupply chamber 4, the retaining device 42 for the supply 40 is provided.Preferably, the gas channel 3 is developed at the same time along withthis retaining device 42. The intake port 3B of the gas channel 3 thenlies higher than the filling level of the supply 40. A conduit can beprovided for this purpose, which is higher than the filling level of thesupply 40 and is inserted through an opening in the separating wall 8.

The supply 40 is heated by its associated heating device 6A and ispartially decomposed. A portion of the SiC sublimates, and theindividual components Si, Si₂ C and SiC₂ of the SiC in the gas phase areconveyed along the direction of the sketched arrows through the gaschannel 3 into the reaction chamber 2. The seed crystal 21 is arrangedon the seed chuck 22 in the lower part of the reaction chamber 2. Asingle crystal 20 grows on this seed crystal 21 as the result of thecrystallization of the SiC out of the gas phase.

By properly configuring the gas channel 3, it is possible to selectivelydirect the SiC gas stream in a desired direction at the crystallizationsurface of the seed crystal 21 or of the single crystal 20. In thedepicted specific embodiment, the center axis 3C of the gas channel 3 isdirected at least nearly perpendicularly to the growth base of the flatseed crystal 21, on which the single crystal 20 grows. Thus, oneachieves a stabilization of the convex phase boundary at the surface ofthe growing single crystal 20, since the SiC is predominantly depositedfrom the vapor phase in the middle and becomes depleted toward theoutside.

Preferably, the two heating devices 6A and 6B are provided separatelyfor the supply chamber 4 and the reaction chamber 2, and can be parts ofa heating device 6 or completely independent of one another. Thismeasure renders possible an easily controllable temperature distributionbetween the SiC supply 4 and the SiC single crystal 20.

The seed crystal 21 and the crystallization surface on the singlecrystal 20 are then retained at a crystallization temperature, which islower than the sublimation temperature at the supply 40, by means of theheating device 6B assigned to the reaction chamber 2, as part of theheating device 6.

The heating device 6 can be designed as a wall system that isinductively coupled to a high-frequency (HF) heating coil (not shown)arranged outside of the system, or also as a resistance heater.Preferably, the heating powers of the two heating devices 6A and 6B areable to be controlled independently of one another to heat the supply 2or the crystallization surface.

In one especially advantageous specific embodiment, means are providedfor supplying a protective gas, of which only one supply line 24 ispreferably depicted, which leads into the reaction chamber 2. By thismeans, the pressure in the reaction chamber 2 and in the supply chamber4 can be additionally adjusted, which then results as the sum of thevapor partial pressures of the components of the SiC in the gas phaseand of the partial pressure of the protective gas. Thus, one caninfluence the sublimation rate of the SiC. Typical pressures lie betweenabout 1 mbar and about 100 mbar and preferably between 1 mbar and 20mbar. Generally, one uses an inert gas, preferably argon, as aprotective gas. Moreover, the vapor pressure of the SiC dependsexponentially on the temperature, so that the sublimation rate and,thus, the crystal growth rate can be adjusted by controlling thesublimation temperature. To control these two essential parameters,pressure and temperature, a controller (not shown) is preferably used,which is electrically connected to the means for introducing theprotective gas and to the heating devices. These two parameters are,thus, able to be precisely adjusted, so that the growth rate can also beexactly controlled. The means for supplying a protective gas into thesupply chamber or the reaction chamber 2 are also preferably used toevacuate the system prior to the deposition process.

In one specific embodiment in accordance with FIG. 2, the supply chamber4 is arranged below the reaction chamber 2. The gas channel 3 is nowpreferably designed as a simple opening in the separating wall 8. Theseed chuck 22 with the seed crystal 21 attached to it is suspended inthe upper part of the reaction chamber 2, so that the seed crystal 21and the single crystal 20 deposited on it are turned toward the openingof the gas channel 3. In this exemplary embodiment, in addition to thetemperature gradient, one makes use of the thermal buoyancy forcesoccurring during the gas conveyance of the sublimated SiC.

The volumes of the supply chamber 4 and of the reaction chamber 2 canvary in magnitude and be selected to be substantially independent of oneanother. Generally, however, the volume of the supply chamber 4 isselected to be larger than that of the reaction chamber 2. Also, thespatial arrangement of the supply chamber 4 and of the reaction chamber2 relative to one another can be selected in any way desired and, inparticular, is not limited to the vertical and axial-symmetricalspecific embodiments pursuant to FIG. 1 or 2.

FIG. 3 depicts a specific embodiment comprising a homogenization chamber5, which is arranged with respect to the gas stream between the reactionchamber 2 and the supply chamber 4 and which is connected via a partialgas channel 11 to the reaction chamber 2 and via another partial gaschannel 7 to the supply chamber 4. Preferably, the three chambers arearranged axially over one another. This makes it possible forcylindrical heating devices to be used. Thus, on its path from thesupply chamber 4 to the reaction chamber 2, the gas stream from thethree components Si, Si₂ C and SiC₂ passes the homogenization chamber 5,which is heated to a temperature T₂ lying generally between thesublimation temperature T₁ and the crystallization temperature T₃. Thevapor pressures of the individual components of the SiC in the gas phasedepend to varying degrees on this temperature T₂. Thus, thestoichiometric proportions Si:Si₂ C:SiC₂ of the three components can bealtered in the homogenization chamber 5 by controlling its temperatureT₂. To independently adjust the temperatures T₁, T₂ and T₃, a heatingdevice 62 is assigned to the reaction chamber 2, a heating device 65 tothe homogenization chamber 5, and a heating device 64 to the supplychamber 4, which are each depicted as resistance heaters.

One special advantage of the process and of the device in accordancewith the invention is the possibility of producing a plurality of SiCsingle crystals simultaneously. In one specific embodiment pursuant toFIG. 4, several systems composed of a reaction chamber 2, a gas channel3, and a supply chamber 4 are provided side by side. In one specificembodiment, systems are depicted in accordance with FIG. 2 with reactionchambers 2 disposed above and supply chambers 4 disposed below. It isalso possible, however, to provide systems in one specific embodiment inaccordance with FIG. 1. A common heating device 61 is assigned to thereaction chambers 2, and a common heating device 60 is assigned to thesupply chambers 4. To control the temperatures in the reaction chambers2 and the supply chambers 4 in a controlled and independent manner, athermal insulation wall 9 is preferably arranged in-between, throughwhich the gas channels 3 pass. The growing single crystals 20 aredepicted in different sizes. The intention here is to indicate that thegrowth rates in the individual systems can deviate from one another,particularly due to varying cross-sections of their gas channels 3.

In another specific embodiment in accordance with FIG. 5, a commonsupply chamber 4 with a supply 40, which is connected in each case by agas channel 3 to the reaction chamber 2, is assigned to a plurality ofreaction chambers 2. Preferably, an insulation wall 9 is again provided.A heating device 60 is assigned to the common supply chamber 4 and,again, a common heating device 61 is assigned to the reaction chambers2. It is also possible to have a plurality of separate heating devicesfor the reaction chambers 2.

Also, in specific embodiments comprising several systems, it is possibleto provide homogenization chambers, which are assigned to individual orto several reaction chambers 2 or supply chambers 4, and/or means forsupplying a protective gas. It is also possible to have the seedcrystals arranged in a common reaction chamber.

Preferred modifications of the grown SiC single crystal 20 are 4H, 6Hand 15R. Preferably, the seed crystal 21 also consists of SiC of thismodification.

All properly heat-resistant materials, in particular high-purityelectrographite, are suitable as materials for the components of thedevice. Moreover, the walls can be provided with preferablypyrolytically produced, heat-resistant coatings.

What is claimed is:
 1. A device for producing single crystals from asolid SiC supply, including a seed crystal having a crystallizationsurface, wherein the device comprises:a) a reaction chamber, wherein theseed crystal is arranged for growing the single SiC crystal out of a gasphase; b) a supply chamber, which is at least partially filled with thesupply of solid SiC; c) a homogenization chamber; d) a first gas channelconnecting the homogenization chamber to the supply chamber; e) a secondgas channel connecting the homogenization chamber to the reactionchamber; and f) at least one heating device for producing SiC in the gasphase from the SiC supply in the supply chamber, for controlling thetemperature in the homogenization chamber, and for adjusting atemperature distribution in the reaction chamber.
 2. The deviceaccording to claim 1, wherein the second gas channel has an exhaust portdisposed directly opposite the seed crystal, so that a center axis ofthe second gas channel is directed at least nearly perpendicularly tothe crystallization surface of the seed crystal.
 3. The device accordingto claim 2, wherein the supply chamber is arranged above the reactionchamber.
 4. The device according to claim 2, wherein the supply chamberis arranged below the reaction chamber.
 5. The device according to claim1, wherein the supply chamber is arranged above the reaction chamber. 6.The device according to claim 1, wherein the supply chamber is arrangedbelow the reaction chamber.
 7. The device according to claim 1, whereina separate heating device is assigned to the homogenization chamber. 8.The device according to claim 1, wherein the supply chamber, thehomogenization chamber, the reaction chamber, and the first and secondgas channels are disposed axially to one another.
 9. The deviceaccording to claim 1, further comprising means for supplying aprotective gas.
 10. A device for producing single crystals from a solidSiC supply, including a plurality of seed crystals, each seed crystalhaving a crystallization surface, wherein the device comprises:a) atleast one reaction chamber, wherein the seed crystals are arranged forgrowing single SiC crystals out of a gas phase; b) at least one supplychamber, wherein each of said at least one supply chamber is at leastpartially filled with the supply of solid SiC; c) at least onehomogenization chamber; d) at least one first gas channel connecting theat least one homogenization chamber to the at least one supply chamber;e) a plurality of second gas channels connecting the homogenizationchamber to the reaction chamber, wherein each second gas channel isdirected to a corresponding seed crystal; and f) at least one heatingdevice for producing SiC in the gas phase from the SiC supply in the atleast one supply chamber, for controlling the temperature in the atleast one homogenization chamber, and for adjusting a temperaturedistribution in the at least one reaction chamber.
 11. The deviceaccording to claim 10, wherein the seed crystals are arranged in acommon reaction chamber.
 12. The device according to claim 11, whereineach of the at least one first gas channels are connected to a commonsupply chamber.
 13. The device according to claim 10, wherein each ofthe at least one first gas channels are connected to a common supplychamber.
 14. The device according to claim 10, wherein the at least onereaction chamber is separated from the at least one supply chamber by athermal insulation wall, and a first heating device is assigned to theat least one reaction chamber, and a second heating device independentof the first heating device is assigned to the at least one supplychamber.